The burning of fuels such as natural gas, oil or coal in power plants, pipeline compressor stations or flares generates polluting emissions. The exhaust from the burning of the fuels is usually evacuated through an exhaust stack or duct, which usually takes the form of a chimney. Most of environmental protection agencies worldwide regulate the emissions from these exhaust stacks or ducts. Standardized methods exist for measuring parameters such as the mass emission rates of molecules such as NOR, CO, CO2, SO2, particulate matter and/or the like.
Most of these standardized methods rely on sampling part of the exhaust stream inside the stack or duct. A probe head is inserted into the stream through a sampling port. Usually, there is more than one port on those stacks that have sampling ports. These sampling ports are used for measuring gas flow velocity, ascertaining that the flow is substantially constant across the entire stack cross section, and verifying that the sampling location is representative of the entire stream. Sampling with the probes is very much localized inside the stream. In some cases, the probe samples the exhaust gas which is routed through tubing and other apparatus to a measurement instrument that is calibrated for concentration measurements of different molecules or particulates. The measurement instruments are usually based on optical principles.
There are also alternative techniques to localized sampling of the exhaust stream. Such alternative techniques, such as integrated path continuous emissions monitoring (IP-CEM) techniques, allow measurements across the entire stack stream and do not require any sampling of part of the stack gas or any routing of the samples to the measurement instrument. However, these alternative measurement techniques also require the presence of ports with mounting flanges. An example of an IP-CEMS method is the US EPA PS-18.
Mounting instruments or inserting probes through sampling ports requires that there be sampling ports on the stack or duct, which is not always the case. A technique is thus required to monitor emissions without the use of sampling ports. In addition, mounting instruments or inserting probes usually has an impact on operations. In order to insert or install probes and instruments, the evacuation of hot and noxious emissions through the stack or duct must be halted, having a detrimental impact on operations. Moreover, having personnel working around the exhaust stacks or ducts and on the premises requires special training and oversight.
Such a remote monitoring method exists for remotely monitoring point sources such as exhaust stacks or ducts. This remote monitoring method relies on a differential absorption light detection and ranging (lidar) apparatus (DIAL) away from the stack or duct. The emissions are allowed to disperse in the atmosphere and form a large plume that is carried by the wind. A pulsed laser is sent across the plume and the backscattered laser light is measured through a receiver telescope. The amount of backscattered light is measured with respect to time after the emission of the laser pulse, which gives a spatially resolved measurement along the beam propagation axis. The amount of backscattered light depends on the attenuation of the laser beam along its axis of propagation which in turns depends on the scattering from particulates and molecules, and on the absorption by molecules. The wavelengths of light for which there is measurable absorption is different for each molecule and constitutes a fingerprint for the molecule. By measuring backscatter light at a number of wavelengths of which at least one wavelength is significantly absorbed by the targeted molecule, a map of the concentration of the molecule can be built through the spatially resolved measurement of the backscattered light along the laser beam propagation axis and by scanning the laser beam across a volume of space. By building a concentration map in a plane perpendicular to the wind direction, and measuring the wind speed, mass emission rates of pollutants can be computed. This is described in detail in VDI 4210, a German standard for emissions measurements using a lidar system. This approach requires that the lidar system be positioned at a relatively large distance from the stack, commensurate with dispersed plume size, and measure small concentration-length products in the dispersed plume, and consequently it will use a large laser with a consequent amount of power and a large receiving telescope and large scanning optics, all of which are mounted on a large mobile platform. This approach is seldom used because there are but a few of these systems that have been built and they are complicated and expensive to use and cannot be driven to many of the remote sites that need to be monitored. And also, they depend on stable wind, in direction and strength. In addition, the spatial resolution is seldom below 5 meters, because of laser pulse length and the necessary large volumes that need to be probed for the detection of the very low concentrations in the dispersed plume. In addition, mixing with the atmospheric air needs to be considered and corrected for.
Therefore, there is a need for an improved method and system for remotely monitoring molecules contained in emissions from an exhaust stack.